1. Field of the Invention
The invention relates generally to power regulation and more particularly to precise and adaptive power regulation of LED driver modules and methods for emergency lighting applications.
2. Description of the Related Art
Traditionally, a switching mode power supply is a voltage regulated converter to maintain constant voltage at the output load or a current regulated converter to maintain constant current at the output load. For emergency light systems the load is the lighting output. Most of today's emergency light systems use LEDs as lighting output for best efficiency. It is well known that LED consumes much less power than any other light sources. Since emergency power lighting is powered by back up battery in an emergency event, consuming as little power as possible is critical to extend the life of the light output. However, the government regulation for emergency lighting requires that in an emergency event, the LEDs light output need to provide a certain minimum lumens or brightness for a minimum period of 90 minutes using the backup battery power for the emergency lighting. The best and most optimal solution to meet the above requirement is to supply constant power to the LEDs from the back up battery during the emergency event. However, providing the sufficient minimum lumens for the required period of at least ninety minutes for a wide range of LED strings and different manufacturers of LEDs by using a backup battery system is a great challenge.
Voltage drops on different LED strings is based on the numbers of LED cells in the series of the LED strings and on the type of LED used in the LED strings. The voltage-current (V-I) characteristic of an LED is not linear. A constant voltage converter can be only used for one particular type of LED or LED strings' load. If a different type or a different make of LEDs are used, then the constant voltage converter is insufficiently able to meet the requirements.
The emergency LED light's lumens is according to the battery capacity used, in terms of power. A conventional emergency LED driver is a current-regulated converter which deliver a constant current over a range of load voltages. This results in LED light's lumens gradually increasing from using more LEDs or gradually decreasing from using less LEDs in the emergency lighting system. Using constant current can also results in battery power not being completely used up during the ninety minutes if the lighting system uses less LEDs. This results in wasted left over capacity of the backup battery and is not cost effective and not optimal. On the other hand, if the same lightning system uses more LEDs, the power from back up battery may not last the required minimum 90 minutes of required output lumens. This results in the failure to meet the safety standards and safety testing (e.g., by UL). Therefore, the best solution is to use a constant power to drive the LEDs output instead of using constant voltage or constant current type. Constant power will solve the issues described above.
In the market today, there are companies manufacturing constant power LEDs driver circuit for the emergency lighting system. As an example, a prior art invention which was patented by this inventor, titled “Constant Power Supply for LED Emergency Lighting using Smart Output Resetting Circuit for no Load Condition” and disclosed in U.S. Pat. No. 9,398,649, focuses on inherent property of constant power in discontinuous conducted mode (DCM) flyback converter. In that invention, the primary side power is regulated without feedback from secondary side, which results in rough power regulation. U.S. Pat. No. 9,398,649 teaches that the output power is 0.5*Lp*(Ip{circumflex over ( )}2)*fsw*η, where Lp is transformer primary winding inductance, Ip is the peak primary inductor's current, fsw is flyback converter's switching frequency and η is converter's efficiency. Assuming Lp, fsw and q are constant, then regulating Ip can regulate output power. However, in the real world, the transformer primary winding inductance's error tolerance is normally 15% or more. In addition, the Ip sensing resistor could have a 1% error tolerance. The frequency is set by RC (resistor-capacitor) network. It is a common knowledge for those skilled in the art that the RC error could be up to 6%. In U.S. Pat. No. 9,398,649, if the converter efficiency is the same for all unit, the power regulation error is affected by the errors of Lp, Ip, and fsw. The actual output power isPo=0.5*(a*Lp)*((b*IP){circumflex over ( )}2)*(c*fsw)*η,where a, b and c are variables. There are factors of these errors of each component. The power error isδP %=a*(b{circumflex over ( )}2)*c−1.
As an example, if the variable a is 1.15, variable b is 1.01 and variable c is 1.06, then the power regulation error is 24.35%. If the variable a is 0.85, variable b is 0.99 and variable c is 0.94, then power regulation error is −21.69%. Thus, in this example, the power regulation error is in the range of −21.69% to 24.35%. Due to the wide error range, this means that in mass production, manual adjustment of each driver's output power is needed and that significantly increases product cost.
The prior art (U.S. Pat. No. 9,398,649) methods for achieving constant power is limited by poor regulation and therefore the need to do manual tuning of each part during mass production which increases cost. In addition, this prior art teachings are also limited to one topology which is the discontinuous conducted mode flyback converter.
The applicant(s) substantially solved the problems outlined above in the related application, U.S. Non-Provisional application Ser. No. 15/959,101 (“'101 application”). However, some potential drawbacks regarding reliability and manufacturing costs may be present in the circuits disclosed in the '101 application. For example, the voltage potential of the LED's load return line of the emergency lighting system (“'101 EM system”) of the '101 application and the voltage potential of the LED's load return line of an external AC driver may be different, resulting in a less reliable emergency lighting system. In other words, the circuit's secondary ground of the '101 EM system and the circuit's secondary ground of the external AC driver may be different, which may cause the external AC driver to shut down due to ground bouncing. To solve the voltage potential difference additional components, such as two relays, may need to be added to the circuit, resulting in higher manufacturing cost.
A second potential drawback could be found in the current sensing circuit of the '101 application. The output current sense signal may have a bigger frequency spike and ripple from the power MOSFET switch and transformer, i.e., the output current sense signal may not be clean. This potential drawback could result in the compensation circuit becoming unstable and cause circuit oscillation, thus the circuit may be less reliable. Another potential drawback found in the '101 application may be that the output current sample is referenced to circuit ground and the analog multiplier IC chip U4 inputs, thus the current sample may need to be isolated. Isolating the current sample may require additional components, and thus more cost and power loss in the circuit.
Lastly, a potential drawback of the '101 application may be that the voltage sample may have worse resolution when the LED load is at the lower end of its output voltage range. Thus, the circuit may be less reliable.
Therefore, there is a need for new and improved power regulation systems and methods for emergency lighting applications that address and solve the problems described above.
The aspects or the problems and the associated solutions presented in this section could be or could have been pursued; they are not necessarily the approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches presented in this section qualify as prior art merely by virtue of their presence in this section of the application.